Re: Sleeping BUG in khugepaged for i586

[Matthew Wilcox]

On Thu, Jun 08, 2017 at 04:48:31PM +0200, Michal Hocko wrote:
> On Wed 07-06-17 13:56:01, David Rientjes wrote:
> > I agree it's probably going to bisect to 338a16ba15495 since it's the 
> > cond_resched() at the line number reported, but I think there must be 
> > something else going on.  I think the list of locks held by khugepaged is 
> > correct because it matches with the implementation.  The preempt_count(), 
> > as suggested by Andrew, does not.  If this is reproducible, I'd like to 
> > know what preempt_count() is.
> 
> collapse_huge_page
>   pte_offset_map
>     kmap_atomic
>       kmap_atomic_prot
>         preempt_disable
>   __collapse_huge_page_copy
>   pte_unmap
>     kunmap_atomic
>       __kunmap_atomic
>         preempt_enable
> 
> I suspect, so cond_resched seems indeed inappropriate on 32b systems.

Then why doesn't it trigger on 64-bit systems too?

#ifndef ARCH_HAS_KMAP
...
static inline void *kmap_atomic(struct page *page)
{
        preempt_disable();
        pagefault_disable();
        return page_address(page);
}
#define kmap_atomic_prot(page, prot)    kmap_atomic(page)


... oh, wait, I see.  Because pte_offset_map() doesn't call kmap_atomic()
on 64-bit.  Indeed, it doesn't necessarily call kmap_atomic() on 32-bit
either; only with CONFIG_HIGHPTE enabled.  How much of a performance
penalty would it be to call kmap_atomic() unconditionally on 64 bit to
make sure that this kind of problem doesn't show on 32-bit systems only?